PLASMA

in physics, an electrically conducting medium in which there are roughly equal numbers of positively and negatively charged particles, produced when the atoms in a gas become ionized. It is sometimes referred to as the fourth state of matter, distinct from the solid, liquid, and gaseous states. The negative charge is usually carried by electrons, each of which has one unit of negative charge. The positive charge is typically carried by atoms or molecules that are missing those same electrons. In some rare but interesting cases, electrons missing from one type of atom or molecule become attached to another component, resulting in a plasma containing both positive and negative ions. The most extreme case of this type occurs when small but macroscopic dust particles become charged in a state referred to as a dusty plasma. The uniqueness of the plasma state is due to the importance of electric and magnetic forces that act on a plasma in addition to such forces as gravity that affect all forms of matter. Since these electromagnetic forces can act at large distances, a plasma will act collectively much like a fluid even when the particles seldom collide with one another. Nearly all the visible matter in the universe exists in the plasma state, occurring predominantly in this form in the Sun and stars and in interplanetary and interstellar space. Auroras, lightning, and welding arcs are also plasmas; plasmas exist in neon and fluorescent tubes, in the crystal structure of metallic solids, and in many other phenomena and objects. The Earth itself is immersed in a tenuous plasma called the solar wind and is surrounded by a dense plasma called the ionosphere. A plasma may be produced in the laboratory by heating a gas to an extremely high temperature, which causes such vigorous collisions between its atoms and molecules that electrons are ripped free, yielding the requisite electrons and ions. A similar process occurs inside stars. In space the dominant plasma formation process is photoionization, wherein photons from sunlight or starlight are absorbed by an existing gas, causing electrons to be emitted. Since the Sun and stars shine continuously, virtually all the matter becomes ionized in such cases, and the plasma is said to be fully ionized. This need not be the case, however, for a plasma may be only partially ionized. A completely ionized hydrogen plasma, consisting solely of electrons and protons (hydrogen nuclei), is the most elementary plasma. in biochemistry, the straw-coloured liquid (including its solutes but excluding the formed elements, such as blood cells and platelets) that constitutes the circulating fluid in animals. Plasma consists of more than 90 percent water, about 7 percent protein, and small percentages of other substances, such as inorganic salts, glucose, and urea and other waste products of metabolism. It serves as a transport system and medium for nutrients, hormones, waste products, and blood cells; helps to maintain blood pressure; distributes heat equally throughout the body; and keeps a steady acid-base balance in the bloodstream and body. Important plasma proteins include fibrinogen, an essential protein necessary for the clotting of blood; albumin, which helps maintain the pressure in blood vessels by its osmotic effect; and a variety of globulins, among which are a hormone that stimulates red blood cell formation and gamma globulin, which is rich in antibodies. See also blood. in mineralogy, semitranslucent, microgranular or microfibrous, semiprecious variety of the silica mineral chalcedony. Its colour, various shades of green, is due to disseminated silicate particles of different kindse.g., amphibole or chlorite. Other properties are those of quartz (see Table 2 under silica mineral). Plasma often has nodules of gray quartz or red jasper (bloodstone) throughout its mass. It has long been used for carvings and mosaics. Localities are India, China, Madagascar, Germany, Brazil, Australia, and Egypt. in physics, an electrically conducting medium in which there are roughly equal numbers of positively and negatively charged particles, produced when the atoms in a gas become ionized. It is sometimes referred to as the fourth state of matter, distinct from the solid, liquid, and gaseous states. When energy (e.g., heat) is continuously applied to a solid, it first melts, then it vaporizes, and finally electrons are removed from some of the neutral gas atoms and molecules to yield a mixture of positively charged ions and negatively charged electrons, while overall neutral charge density is maintained. When a significant portion of the gas has been ionized, its properties will be altered so substantially that little resemblance to solids, liquids, and gases remains. A plasma is unique in the way in which it interacts with itself, with electric and magnetic fields, and with its environment. A plasma can be thought of as a collection of ions, electrons, neutral atoms and molecules, and photons in which some atoms are being ionized simultaneously with other electrons recombining with ions to form neutral particles, while photons are continuously being produced and absorbed. Scientists have estimated that more than 99 percent of the matter in the universe exists in the plasma state. All of the observed stars, including the Sun, consist of plasma, as do interstellar and interplanetary media and the outer atmospheres of the planets. Although most terrestrial matter exists in a solid, liquid, or gaseous state, plasma is found in lightning bolts and auroras, in gaseous discharge lamps (neon lights), and in the crystal structure of metallic solids. Plasmas are currently being studied as an affordable source of clean electric power from thermonuclear fusion reactions. Additional reading For a general audience an early work that is still of considerable use is Lev A. Arzimovich (L.A. Artsimovich), Elementary Plasma Physics (1965; originally published in Russian, 1963). A more recent work at the same level is Yaffa Eliezer and Shalom Eliezer, The Fourth State of Matter: An Introduction to the Physics of Plasma (1989). A broad perspective on plasma physics is provided by National Research Council (U.S.), Panel on the Physics of Plasmas and Fluids, Plasmas and Fluids (1986). For applications, see National Research Council (U.S.), Panel on Plasma Processing of Materials, Plasma Processing of Materials: Scientific Opportunities and Technological Challenges (1991). Descriptions of fusion-energy options with a discussion of plasma aspects are included in Roger A. Hinrichs, Energy (1992); and Ruth Howes and Anthony Fainberg (eds.), The Energy Sourcebook (1991). A historical treatment with numerous illustrations dealing with the aurora is Robert H. Eather, Majestic Lights (1980). More advanced mid-college-level texts include Francis F. Chen, Introduction to Plasma Physics and Controlled Fusion, vol. 1, Plasma Physics (1984); and Michael C. Kelley and Rodney A. Heelis (Roderick A. Heelis), The Earth's Ionosphere: Plasma Physics and Electrodynamics (1989). Michael C. Kelley